WASHINGTON (Reuters) - Hillary Clinton on Friday released her 2015 tax returns, which showed the Democratic presidential nominee and her husband had $10.75 million in income that year and paid an effective federal tax rate of 34.2 percent.

In 2015, the Clintons made $1 million in charitable contributions, mostly to the Clinton Foundation; former President Bill Clinton brought in nearly $5.3 million in speaking fees; and the former secretary of state reported income of $3 million from publisher Simon & Schuster for her book on her tenure at the State Department.

Clinton's running mate, U.S. Senator Tim Kaine of Virginia, along with his wife, Anne Holton, released 10 years of tax returns. They paid a federal effective tax rate of 20.3 percent in 2015.

"Hillary Clinton and Tim Kaine continue to set the standard for financial transparency," Clinton campaign aide Jennifer Palmieri said in a statement. "In stark contrast, Donald Trump is hiding behind fake excuses and backtracking on his previous promises to release his tax returns."

It is customary for U.S. presidential candidates to make their tax returns public, although they are not required by law to do so.

Clinton's tax returns have been made public, in some form, every year since 1977.

Trump, a New York businessman, and his lawyers have cited an audit by the Internal Revenue Service as a reason for his refusal to release his returns. Trump also has said his taxes are no one's business and that they reveal little.

The IRS has said Trump can release his tax returns even while under audit.

On Thursday, Trump's special counsel, Michael Cohen, told CNN he would not allow Trump to release them until the audits are complete.

RELATED: 41 powerful photos of Hillary Clinton's storied career

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41 powerful photos of Hillary Clinton's storied career

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41 powerful photos of Hillary Clinton's storied career

Hillary Clinton, First Lady of the United States, throws a thumbs-up during a presidential election victory celebration in 1992. Her husband might be doing the same for her, 24 years later.

(Photo by Win McNamee / Reuters)

Here she is talking to kids at the Children's Memorial Hospital in Chicago, October 21, 1993.

(Photo by Sue Ogrocki / Reuters)

As First Lady, she did a lot of traveling, like to Bosnia in 1996 to meet US soldiers.

(Photo by Win McNamee / Reuters)

In 1997 she visited Goree Island, Senegal, with her daughter Chelsea. Here they are peering out from the Door of No Return, a former slave trading center, as a soldier stands guard.

(Photo by Win McNamee / Reuters)

Nelson Mandela showed Clinton and Chelsea the cell in which he was held for 27 years at the Robben's Island prison off the coast of Cape Town, March 20, 1997.

(Photo by Win McNamee / Reuters)

Here she is with Bill in the Oval Office, chatting with a bunch of kids awaiting adoption.

(Photo via Reuters)

Sporting similar power suits and haircuts, Hillary met with Diana, Princess of Wales, at the White House in 1997.

(Photo via Reuters)

No one can claim that she doesn't get her hands dirty. Here's Clinton building a home as a volunteer for Habitat for Humanity in Pikeville, Kentucky, in 1997.

(Photo via Reuters)

She's always been an advocate for affordable health care as well as women's rights. In 1998, Clinton gave a speech at Beijing Medical University about improving health care in China, particularly folic acid deficiencies in pregnant mothers.

(Photo by Natalie Behring / Reuters)

Like any presidential nominee, she's good with babies. She found this one during a visit to a children's home in the Dominican Republic in 1998.

(Photo via Reuters)

Hillary kneels before the grave of US Private Celia Goldberg, who was killed in Tunisia during World War II, at the North Africa American Cemetery, outside Tunis, in 1999.

(Photo via Reuters)

In 2000, she announced her candidacy for New York's Senate seat.

(Photo via Reuters)

Later that year, Clinton held an event at the White House on preventing potential harm to children from defective products.

(Photo via Reuters)

She won that New York Senate seat on November 7, 2000. Definitely not a "low energy" candidate, based on this picture.

(Photo via Reuters)

Days after 9/11, she took a tour of the World Trade Center disaster site.

(Photo via Reuters)

Here Clinton is giving kids from Manhattan's Colombia Grammar and Prep school a tour of her office on Capitol Hill in 2003.

(Photo by Reuters)

She wrote a book titled "Living History."

(Photo by Chip East / Reuters)

In 2003, all Clinton could do was smirk on "The Tonight Show with Jay Leno" as Leno shows her a tabloid newspaper article about her hooking up with an alien.

(Photo via Reuters)

A power woman power lunches with US troops in Bagram Airbase, north of Kabul, in 2003.

(Photo via Reuters)

Here she's listening to Defense Secretary nominee Robert Gates testify before the US Senate Armed Services Committee during his confirmation hearing on Capitol Hill in 2006.

(Photo by Jim Young / Reuters)

Hillary and Bill pay their respects to the late former President Gerald Ford in 2007.

(Photo by Jim Young / Reuters)

This is her first presidential campaign's website in 2007. It's pretty low-tech.

(Image via Reuters)

Back in 2007, she was running against Obama for the Democratic Presidential nomination.

(Photo by Lisa Hornak / Reuters)

Their body language says more than a thousand words.

(Photo by Steve Marcus / Reuters)

She spent most of 2008 on the campaign trail.

(Photo by Chris Keane / Reuters)

She's nothing if not ecstatic.

(Photo by Bradley Bower / Reuters)

It was a close race, but she had to endorse presumptive Democratic presidential nominee Barack Obama at the National Building Museum in Washington, June 7, 2008.

(Photo by Jason Reed / Reuters)

A gracious loser, she waved to delegates at the 2008 Democratic National Convention in Denver, August 26, 2008.

(Photo by Eric Thayer / Reuters)

Despite losing the nomination, her and Obama found that they had a lot in common. He later made her Secretary of State.

(Photo by Jim Young / Reuters)

Secretary of State Clinton and President Obama tour the Sultan Hassan Mosque in Cairo in 2009.

(Photo by Larry Downing / Reuters)

A North Korean soldier looks in through the window as Hillary tours the Demilitarized Zone in Panmunjom, South Korea, in 2010.

(Photo by Cherie Cullen/Defense Department photo via Reuters)

This is the badass pic that launched a thousand memes. Hillary looks cool as a cucumber checking her phone on a military C-17 plane to Libya in 2011.

(Photo by Kevin Lamarque / Reuters)

Here she is in the Situation Room with Obama and Vice President Joe Biden, along with members of the national security team, in 2011. They just received news on the mission against Osama bin Laden.

(Photo by White House/Pete Souza via Reuters)

They say the second time's a charm: here is Clinton delivering her "official launch speech" at a campaign kick off rally on Roosevelt Island in New York City, June 13, 2015.

(Photo by Lucas Jackson / Reuters)

Peek-a-boo: Hillary sizes up her audience at a campaign launch party at Carter Hill Orchard in Concord, New Hampshire, on June 15, 2015.

(Photo by Brian Snyder / Reuters)

Hillary takes the stage to speak during the Scott County Democratic Party's Red, White and Blue Dinner at the Mississippi Valley Fairgrounds in Davenport, Iowa, January 23, 2016.

(Photo by Scott Morgan / Reuters)

Bill is thrilled as his wife speaks at a campaign stop in Davenport, Iowa, in January 2016.

(Photo by Brian Snyder / Reuters)

A woman of the people, she hugged Brana Marancic, an employee of Caesar's Palace in Las Vegas, Nevada, in February 2016. They appear to be in a storage closet.

(Photo by Jim Young / Reuters)

It was a fierce race between Clinton and Bernie Sanders

(Photo by Mike Segar / Reuters)

Here she is speaking to supporters at her New York presidential primary night rally in Manhattan, April 19, 2016.

(Photo by Mike Segar / Reuters)

She did it! Hillary is officially the Democratic presidential nominee, and the first woman in the 240-year history of the US to lead a major party's presidential ticket.

(Photo by Carlos Barria / Reuters)

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Trump's critics, including 2012 Republican presidential candidate Mitt Romney and other fellow Republicans, have said his refusal raises questions about his net worth, his charitable contributions, his business dealings and various other ties, including with Russia.

Clinton has pounced on the issue, releasing an online video on Friday highlighting high-profile Republicans urging Trump to release his taxes. On Thursday, she raised the issue during an economic speech in Michigan.

"He refuses to do what every other presidential candidate in decades has done and release his tax returns," she told the crowd.

Politico has reported that Trump paid zero to very little taxes for two years in the 1990s, and a New York Times business columnist on Friday quoted a number of tax lawyers and accountants saying that could still be the case.

Federal tax rates have become an issue in the presidential election. Clinton has endorsed a rule named after billionaire investor Warren Buffett that would ensure those making more than $1 million a year pay a tax rate of at least 30 percent. President Barack Obama also backs the proposal.

At a Clinton rally last week in Omaha, Nebraska, Buffett, whose Berkshire Hathaway <BRKa.N> conglomerate is based there, challenged Trump to meet and exchange tax returns. Buffet said that he too was under IRS audit and Trump is "afraid" not of the tax-collection agency but of voters.

Clinton's campaign has released tax returns going back to 2007. The Clintons, who now live in Chappaqua, New York, paid an average effective federal tax rate of about 32 percent from 2007 to 2014 and an effective combined tax rate of approximately 40.5 percent.

The Clintons' 2015 return showed that, unlike most Americans, just $100 of their income came from wages. Their main investment was a low-cost index mutual fund, and the Clintons reported dividend and interest income of $109,000.